The 3rd Generation Partnership Project (3GPP) long term evolution (LTE), which is an advanced version of Universal Mobile Telecommunications System (UMTS), is specified in the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink and uses Single Carrier-frequency division multiple access (SC-FFDMA) in an uplink. The 3GPP LTE adopts MIMO with up to four antennas. Recently, 3GPP LTE-Advanced (LTE-A), which is an evolution of the 3GPP LTE, is under discussion.
As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, a physical channel in the 3GPP LTE/LTE-A is divided into a downlink channel (i.e. Physical Downlink Shared Channel (PDSCH) and Physical Downlink Control Channel (PDCCH), and an uplink channel (i.e. Physical Uplink Shared Channel (PUSCH) and Physical Uplink Control Channel (PUCCH).
In order to reduce the interference caused by uplink transmission between terminals, it is important for a base station to maintain uplink time alignment for a terminal. A terminal may be located in an arbitrary area within a cell, and a reaching time (the time that an uplink signal transmitted by a terminal takes to reach a base station) may vary depending on the position of each terminal. The reaching time of a terminal located at a cell edge is longer than that of a terminal located at the middle of a cell. On the contrary, the reaching time of a terminal located at the middle of a cell is shorter than that of a terminal located at a cell edge.
In order to reduce the interference between terminals, it is necessary for a base station to arrange a schedule to allow uplink signals transmitted by terminals in a cell to be received within each time boundary. A base station is required to appropriately adjust the transmission timing of each terminal depending on the situation thereof, and this adjustment is called uplink time alignment. A random access process is a process for maintaining the uplink time alignment. A terminal obtains a time alignment value (or timing advance (TA)) through the random access process, and then, applies the time alignment value so as to maintain the uplink time alignment.
Recently, in order to provide a higher data transfer rate, a plurality of serving cells has been introduced. However, under the assumption that frequencies between serving cells are adjacent or propagation characteristics between serving cells are similar, the same time alignment value has been applied to all serving cells.
In existing wireless communication systems, uplink transmission is designed in consideration of only the same time alignment value. However, since serving cells having different propagation characteristics are allocated in some cases, uplink transmission needs to be designed in consideration of having different time alignment values between cells.